This application claims the priority of Korean Patent Application No. 2003-62830, filed on Sep. 8, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a light emitting device and a method of manufacturing the same, and more particularly, to a light emitting device having a reflective electrode structure wherein ohmic characteristics are improved and a method of manufacturing the light emitting device.
2. Description of the Related Art
It is important to construct an ohmic contact structure between a semiconductor and an electrode in manufacturing a light emitting device such as a light emitting diode and a laser diode employing a nitride semiconductor, for example, a gallium nitride (GaN) semiconductor. Presently, commercially available GaN light emitting devices are mostly constructed on a sapphire (Al2O3) substrate.
Such GaN light emitting devices are classified into top-emitting light emitting diodes (TLEDS) and flip-chip light emitting diodes (FCLEDS).
TLEDS emit light through an ohmic contact layer in direct contact with a p-type clad layer while the low electric conductivity of the p-type clad layer provides current injection through a transparent ohmic contact layer having a low ohmic value without obstructions.
TLEDS generally have a structure wherein a nickel (Ni) layer and a gold (Au) layer are successively stacked over a p-type clad layer.
It is known that a Ni layer forms a semi-transparent ohmic contact layer having a specific contact resistance of 10−3 to 10−4 Ωcm2 through annealing in O2 ambient.
The low specific contact resistance of the semi-transparent ohmic contact layer is formed between a Ni layer and a GaN forming a p-type clad layer and between the Ni layer and an Au layer when an annealing at a temperature of 500–600° C. is performed in O2 ambient, thus resulting in lowering Schottky barrier height (SBH) and easily providing holes, being multiple carriers, onto around the surface of the p-type clad layer. Therefore, the effective carrier concentration in the vicinity of the p-type clad layer surface increases.
A Ni layer and a Au layer formed over a p-type clad layer show characteristics of ohmic conduction through annealing, wherein the reactivation process removing compounds among Mg—H metals and increasing the concentration of Mg dopant on the surface of GaN makes the effective carrier concentration more than 1019 cm−3 on the surface of the p-type clad layer and causes a tunneling conduction between the p-type clad layer and the ohmic contact layer containing NiO.
TLEDS employing a semi-transparent electrode thin film made of Ni and Au have low light efficiency, thus making it difficult to realize a light emitting device having a large capacity and high brightness.
Recently, there is a need for developing FCLEDS using silver (Ag), aluminum (Al), rhodium (Rh), etc. which are spotlighted as materials for forming a high reflection layer, in order to realize a light emitting device having a large capacity and high brightness.
These metals have high reflective efficiency and thus provide high light emitting efficiency. However, the metals have small work function, resulting in making the life span of a light emitting device short because of the difficulty to form an ohmic contact having a low ohmic value and failing to provide a stable light emitting device because of the poor adhesive property to GaN.
To solve these problems, research on the structure of ohmic contact having a low ohmic value and high reflective efficiency are now under way.
For example, structures wherein indium oxide (In2O3) is stacked over a p-type clad layer have been disclosed in US patent publication Nos. 2003-0143772A1 and 2002-0179914A1. However, the structures have a high ohmic value.